<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
        ++a ; //  先把a = a +1 表达式返回a的值
        a++ ; // 先返回变大时的值 在 a = a+ a;
        var a = 1;  var b = ++a + ++a;
        var a = 1;  var b = a++ + ++a;
        
        // 逻辑运算符
        // &&  || 
        var b = "abc" && "bcd"; 
          
    </script>
</body>
</html>